High-pressure rolling of workpieces

ABSTRACT

A die for drawing tubular workpieces has its orifice defined by a ring of small rollers which are rotatable to work the workpieces around their circumference. Means may be present to straighten the tube and the die may include means to make the wall thickness of the workpiece uniform.

111111111116 @tmtes Mare/11171 1116111 1 Feb, 22, 11972 [54]lHlllGlHI-lilRlEfifilUllklE RUILILHNG 60 1' 1,596,751 8/1926 Millspaugh..72/126 X W/URKPMEQES 1,665,915 4/1928 Ekman 1 1 1 ..72/126 X 2,375,1085/1945 Hopkins ..72/126 [72] Inventor: ,lacob Marcovitch, Johannesburg,Republic 2 07 971 10/1957 Gamer et 1 72 9 OfSourh Africa 3,387,4776/1968 Shupper ..72/126 11 3,473,358 10/1969 Marcovitch 1 ..72/126 X[73] Assgnee' fig? Ammm Vaduz Llechte 1,184,972 5/1916 Lloyd 1 ..72/1261,530,381 3/1925 Leedom ..72/126 [22] Filed: Dec. 115, 1969 1,774,2318/1930 Harnsberger ..72/126 X 1,970,061 8/1934 Stanfield et al. ..72/126[21] flmjm 2,383,214 8/1945 Prout ..72/126 x R E Red U-S'A a D t2,522,257 9/1950 Curtis 1 ..72/126 e a W a m 2,546,756 3/1951 KI'IOWHCOH..72/126 1 Continuation of N 7, l y 1 aban- 3,160,036 12/1964 Ernestus..72/110 )4 doned. 3,449,812 6/1969 Hauschke et a] ..72/126 X [30]Foreign Application Priority 1 am Primary Examiner-Milton S. Mehr May 5,1966 Republic 616611111 A1ric11.....66/26|0 MOWPYOUY'E Tlmmpw Aug. 17,1966 Republic ofSouth Africa. .66/f1 911 [52] 11.5. C1 72/16, 72/87,7722711205, [57] MEMWACT [51] int. Cl. ..lElZllb 19/03, 1321b 3/06 A diefor drawing tubular workpieces has its orifice defined by [58] lFielrliof Search ....72/113, 1 12, 1 10, 107, 102, a ring of small rollerswhich are rotatable to work the work 72/98, 126, 96, 100, 87, 194, 191pieces around their circumference. Means may be present to straightenthe tube and the die may include means to make the [56] lRellereneeslCi/tetll wall thickness of the workpiece uniform.

UNITED STATES PATENTS 27 Claims, 15 Drawing Figures 447,582 3/1891Robertson ..72/96 1,072,001 9/1913 Haas PATENIEBFEBZQ m2 31.8% ,ABB

swan M nr 5 IIIGl'rlI-IPItlESSIJIklE ROLLING F WGIIKIIIIPIIEQES Thisapplication is a continuation of application Ser. No. 634,947, filed MayI, l967 and now abandoned.

This invention relates to the treatment of tubular workpieces, by whichis meant operating on them to produce a tube of uniform outer or innerdiameter, or both, or of uniform wall thickness.

The object of the invention is to provide apparatus which permits tubeto be produced to a high standard of precision which is not attainableby conventional techniques, and at a cost which compares favorably withthe cost of such techniques.

Conventional methods employ dies through which the material is drawn orextruded. Apart from inaccuracy due to die wear, there is always theproblem of maintaining the core of the die in correct alignment with theshell. Without such alignment, wall thickness cannot be controlled, andit is common experience that wall thickness can vary by as much as 20percent in commercial tubing.

To avoid these difficulties and defects, the present invention makes useof the concept of subjecting the tube to a rolling operation, internallyor externally, carried out by a series of small rollers arranged torotate and to profile the tube as they roll in line contact with it, byapplying progressively around the circumference of the tube pressureover a very small area, so that the intensity of pressure is very high.

According to the invention, apparatus to put this concept into practicaleffect consists, broadly, of a die through which the tubular workpieceis adapted to be drawn, the die orifice being, at least partly, definedby a ring of small rollers; means to cause rotation of the rollersaround the circumference of a workpiece in the orifice, and means topass the workpiece through the orifice. The circumference may be theexternal or the internal circumference.

Further according to the invention, the rollers are individually skewed"to the axis of the orifice, so that, as they turn, they tend to draw theworkpiece through the throat. The transport of the workpiece may beachieved solely by this means, or axial force may be applied externallyto the workpiece.

Also according to the invention, the die includes a series of rings ofrollers, spaced apart along the length of the orifice, to reduce theeffective diameter of the orifice, so that the workpiece isprogressively treated as it advances through the orifice.

Several embodiments of the invention are illustrated in the accompanyingdrawings, in which:

FIG. I is a vertical section through one from of die,

FIG. 2 is a fragmentary view, in section, of a form of die in which therollers are arranged to skew.

FIG. 3 is a similar view of another embodiment in which the rollers aremounted on stub axles,

FIG. 4 is a similar view, showing rollers that are pinned in a holder,

FIG. 5 is a similar view of rollers mounted in a cage,

FIG. 6 is an end view showing the rollers backed by other rollers,

FIG. '7 is a fragmentary view showing the arrangement of FIG. 6 indetail,

FIG. 8 is a fragmentary end view partly in section showing a tubularworkpiece being internally worked,

FIG. 9 is a section on the line 9-9 of FIG. d,

FIG. III is an end view of another embodiment for internal working of atube,

FIG. Ill is a side view of the preferred form of roller,

FIG. I2 is a fragmentary vertical section showing a means to straightenas well as work a tube internally,

FIG. I3 is a vertical section through a machine for internally treatinga tube to obtain a uniform wall thickness,

FIG. 14!- is a similar view, but showing a tube being externally treatedfor the same purpose, and

FIG. 15 is an end view showing a tube being treated simultaneously,internally and externally.

In FIG. I, the die consists of a body III having an orifice I2 that isstepped. Inside the orifice is a series of spaced grooves MI, each ofwhich constitutes a race for a series of small rollers 16 which line theorifice to provide the throat through which a tubular workpiece lid isdrawn. For the required relative rotation between the tube 113 androllers I6, the body III may be rotated, or the tube llfi may be rotatedas it is passed through the throat.

Each roller I6 is convex in profile. If the rollers are square on to thetube, external means must be provided to draw the tube through thethroat. Any means suitable for this purpose may be used, for instance achuck which: grips the leading end of the tube, or a capstan aroundwhich the tube is coiled. However, it is preferable to arrange therollers so that they act to draw the tube through the die, or at leastassist in doing so. To this end, the rollers are arranged obliquely tothe axis of the throat, so that they form a sort of Archimedean screw todrive the tube. This arrangement is described later.

It will, of course, be appreciated that, for a small reduction ofdiameter, or for soft material, it may not be necessary to provideseveral rings of rollers: one ring may be sufficient.

In FIGS. I and 2, the rollers are arranged to roll on the floor of thechannels M. In FIG. 3, each roller is provided with stub axles 241) thatroll on the lands 22 of the grooves.

The rollers may be free within the grooves in the sense that they areheld in the grooves only by the: presence of the tube Id. In some cases,this may be disadvantageous since, when the throat is empty, the rollerswill fall out of it. Accordingly, in FIGS. I and 5, the rollers are heldpositively within the grooves, in FIG. d by being mounted on pins 2dthat are journaled within holes in the sidewalls of the grooves, and inFIG. 5 by being mounted within cages 26 with which stub axles 23 on therollers are engaged.

In FIGS. 6 and i, the rollers I6 are backed by rollers 30 which havestub axles 32 that are joumaled in the sidewalls of the groove 34. Therollers 30 are circumferentially grooved at 36, and the groove serves asa guide for the roller I16. As will be seen in FIG. 6, the series ofrollers I6 is intercalated with the series of rollers 30, so that eachroller I6 is backed by two rollers 30.

In FIGS. 4i, 5, b and 7, the rollers may be permanently kept in a skewedposition by skewing their mountings.

Thus far, the embodiments described concern the profiling of the outersurface of tubular workpieces, to provide a uniform outer diameter and agood finish. If the wall thickness of the tube is not uniform, itremains so. In the embodiments that are illustrated in FIGS. ll, 9, It)and I2, it is the inner surface of the tubular workpiece that istreated.

In FIGS. 8 and 9, the tubular workpiece 3% is caused to travel axiallythrough a throat defined between juxtaposed rollers 10 that rotate aboutfixed axes. Inside the throat, and within the cavity of the tube is arod I2 coaxial with the tube, surrounded by a tubular holder 44 slottedto accommodate rotatably a series of rollers 416 which rotate abouttheir axes, which are kept in a fixed position relative to the rollers4M), as shown in FIG, d, by the holder Ml.

In operation, the tube 3% is offered to the throat and is drawn orpropelled through it. The rollers III, or one or two of them, is or arerotated, which imposes rotation on the tube 3%. The rod 412 issimultaneously rotated in the opposite direction. This causes therollers do to rotate. As the tube advances between the rollers as andthe rollers ill, a uniform internal diameter is imposed on it by therollers M6, the tube being buttressed by the rollers II This operationsizes the internal and external diameters simultaneously. Preferably,the tube is initially slightly undersize in these diameters.

Tubes in their raw state are almost invariably bent, especially whenthey come in long lengths. The arrangement of FIG. I2 is intended to beof use in straightening them and slightly elongating them. The raw tube3% is traversed past a multiroller system, which may be large rollers asshown at li) in the top part of the figure, or a series of needlerollers 56 as seen in the lower part. Within the traversing tube It isan assembly of rollers 48, 50 which may be skew mounted, in advance ofwhich is a plunger 52 and to the rear of which is a second plunger 53 ofsomewhat greater diameter. The assembly is rotated by a stem 4:2 fastwith the rear plunger 53, the rotation being transmitted to the innerrollers 68, 50 and the front plunger through the stem 42 around whichthe inner rollers 48, 50 revolve.

The tube 38 is drawn through the throat between the inner rollers andthe outer rollers. The front plunger 52 is a sliding fit within the tubecavity. The inner rollers reduce the wall thickness while straighteningthe tube, and the rear plunger 53 is a sliding fit within the increasedinternal diameter of the tube. For the sake of clarity, this increasehas not been illustrated in FIG. 12.

Pressure oil may be fed to the outer surfaces of the plungers 52, 53,through a bore 55 in the stem 42 and through grooves 57 in the plungersin communication with the bore 55.

In FIG. 10, an arrangement such as in FIGS. 8 and 9 is shown, but inplace of a few inner rollers there is a large number of needle rollers54 between the inner surface of the tube 38 and the outer surface of acore 56. The rollers 54 are constrained against endwise movement byengagement in grooves in the core 56 but need not otherwise beconstrained, as they are held in position by the tube. There aresufficient rollers 54 to line completely the throat defined between thetube and the core.

In FIG. 15, the same system of needle rollers 54 is seen, but the outerrollers 40 give place to a large number of needle rollers 58 whichsurround the tube and which are contained within the bore of a hollow,cylindrical holder 60. The rollers, 58, like the rollers 54, areconfined against axial movement by being located within annular groovesin the holder 60, but need not otherwise be constrained. As the throatis defined between two sets of rollers that are buttressed againstradial displacement, so that the throat is and remains uniform, passageof the tube 38 through the throat imposes on the tube prescribed innerand outer diameters.

In FIGS. 13 and 14, embodiments are shown which produce concentric,straight tubes from commercial tubing which, as is usual, is botheccentrically apertured and bent. In FIG. 13, a tube 62, which has beensized as to outer diameter by the apparatus or otherwise, is placedwithin a housing 64 having a bore 66 in which the tube is a close fit.The bore 66 is, at 68, reduced to the exact diameter to which it isdesired to bring the inner diameter of the tube. A number of rollers 70,arranged for orbiting in a circle, which is exactly the diameter of thereduced bore of the housing, is mounted on a mandrel 72 which is asliding fit in the reduced bore 68 and which projects into the bore 66to define therewith an annular passage. The rollers 70 are passedthrough the reduced bore by advancing the mandrel 72, which issimultaneously rotated, by means such as a belt 74 which engages apulley 76 slidably mounted on a splined stem 78 projecting rearwardlyfrom the mandrel 72. As the rollers 70 advance into and through thetube, they orbit and roll out any eccentricity in the bore of the tube,which is removed from the machine straight, with uniform wall thickness,and well-finished internally. The strength of the tube will be enhancedby the rolling operation.

In FIG. 14, the tube 80 is first finished as to its inner diameter,either by the means of the invention or otherwise. Thereafter, it isimpaled upon a mandrel 82 and inserted into an annular passage definedby the mandrel 82 and an enlarged bore or counterbore 88 of acylindrical housing 86. The mandrel 82 has a shoulder 84 against whichthe end of the tube abuts, and which marks the division between thelength of mandrel that enters the tube and a part of diameter exactlyequal to that to which the outer diameter of the tube is to be brought.This larger diameter is used as a guide over which slidably passes acylindrical housing 86 that carries, with a counterbore 88, a series ofsmall rollers 90 which orbit the tube, and define a circle of exactlythe diameter to which the tube is to be brought. The housing is causedto traverse the tube and at the same time is rotated by means such as agear wheel 92 that engages teeth 94 on the periphery of the casing. Thegear wheel 92 is rotated through a shaft 96 driven by a belt 98 passingaround a pulley 100 on the shaft. Rotation of the housing causes therollers to orbit around the tube and to profile the tube betweenthemselves and the mandrel 82 to the prescribed outer diameter, at thesame time rolling out eccentricities in the bore to produce a uniformwall thickness.

The mandrel in FIG. 14, or the bore 66 in FIG. 13 might, of course, beprofiled such as by splines or other formations to produce acomplementally profiled surface in the finished tube.

The shape of the rollers that impose the profile upon the workpiece is,in all embodiments of the invention, of importance. A convex or barrelshape is essential for internal profiling. The best form of roller isillustrated in FIG. ll, in which it will be seen that the convexityconsists of an annular swage 102 at the middle of the roller. Forexternal rolling, the rollers may be convex, cylindrical or even concaveif the rollers are obliquely mounted.

The rollers may be obliquely mounted to promote travel of the tuberelatively past them, or it may be desirable that the rollers be somounted that they take up their own orientation as the operationproceeds. To this end, the rollers may be floatably mounted as in FIG.2, in the sense that they are permitted a measure of play which permitsthem to swivel to balance the forces acting on them.

It will be observed that in all embodiments the rollers are backedeither longitudinally or at both their ends against thrust from theworkpiece. This allows high-pressure intensity to be exerted over asmall area of the workpiece while the rollers are fully supported, and alarge force may be applied to urge them towards the workpiece.

I claim:

1. Apparatus for rolling a tubular workpiece comprising structuredefining an annular throat and within the throat an annular series ofnonspherical rollers, each of which has its center portion of greaterdiameter than the diameter of its axially outer portions, said throatcomprising at least one backing member having a surface which is asurface of revolution on which surface at least said outer portions ofthe rollers roll in line contact, whereby in use a tubular workpiecepassing through the throat with relative rotation between the workpieceand the rollers about the axis of the workpiece undergoes deformation ofa surface thereof.

2. The apparatus of claim 1 further comprising means for causingrelative rotation between the rollers and a workpiece.

3. The apparatus of claim 1 further comprising means for advancing theworkpiece through the throat.

4. The apparatus of claim 1 wherein the rollers are arranged so as inuse to act on the outside of the workpiece.

5. Apparatus as claimed in claim 4, and a mandrel located within thethroat to support the inside surface of the workpiece.

6. Apparatus as claimed in claim 5, in which the surface of said mandrelis contoured.

7. The apparatus of claim ll wherein the rollers are arranged so as inuse to act on the inside of the workpiece.

8. Apparatus as claimed in claim 7, in which the throatdefiningstructure is contoured.

9. The apparatus of claim 1 wherein the rollers are arranged with theiraxes substantially parallel with the axis of the throat.

10. The apparatus of claim 1 in which the rollers are arranged withtheir axes oblique relative to the axis of the throat.

11. The apparatus of claim 1 including a number of annular series ofrollers spaced apart along the length of the throat.

12. The apparatus of claim 111 in which the diameters of the circlesencompassed respectively by the series of rollers decrease along thelength of the throat.

13. The apparatus of claim I in which the backing member is formed withan annular groove for said series of rollers.

14. The apparatus of claim 13 in which the backing member is bored toprovide the throat and said groove is provided within the wall of thebore.

iii. The apparatus of claim 7 in which the backing member is a stemadapted to enter the workpiece.

lid. The apparatus of claim 1155 in which the structure is a housingwith a cylindrical aperture which encloses the workpiece.

ll. The apparatus of claim id in which the structure is a se ries ofrollers parallel with and mounted around the workpiece.

lb. The apparatus of claim lid in which the backing member is bored tothe outer diameter to which the workpiece is to be brought and iscounterbored to receive the rollers.

l9. Apparatus for treating a hollow workpiece, comprising first andsecond means defining between them an annular aperture and an annularseries of rollers in the aperture, said first means including at leastone backing member having a surface which is a surface of revolution onwhich surface at least the axially outer portions of the rollers aresupported in line contact, the workpiece and said first means beingaxially and rotatively movable relative to each other to cause therollers to traverse a periphery of the workpiece, said rollers beingfreely supported on said backing member for orbital movement about saidbacking member whereby upon rotative movement of said first means andworkpiece relative to each other, the line of contact between eachroller and the backing member rotates about the periphery of the backingmember and a surface of the workpiece undergoes deformation.

Lilli). Apparatus as claimed in claim 19, said rollers being elongatedin their axial direction and having a center portion which contacts theworkpiece and is of greater diameter than the endwise outer portions ofthe rollers which do not contact the workpiece, the total length of saidline of contact between said backing member and the rollers beinggreater than the length of said center portion which contacts theworkpiece.

21. Apparatus as claimed in claim lit, in which said hacking membercomprises a mandrel that extends into the workpiece to cause the rollersto act on the inner surface of the workpiece, said second meanscomprising an elongated circumferentially continuous wall that acts onthe outer surface of the workpiece.

22. Apparatus as claimed in claim ill, in which the mandrel is of adiameter equal to that required of the workpiece, the mandrel having asliding lit in an elongated bore coaxial with the circumferentiallycontinuous wall and formed in a rigid housing.

23. Apparatus as claimed in claim Ml, in which each of the rollers hasstub axles on opposite axial ends of said center portion, the stub axlesof each roller having a combined axial length greater than that of saidcenter portion, at least the stub axles of the rollers being supportedin line contact on said backing member.

M. A method of rolling a workpiece having at least one surface which isa surface of revolution, the method comprising:

a. rolling the said surface at high pressure by the median portion ofeach of a plurality of rollers, each roller being arranged with a majorcomponent of the direction of its axis parallel to the axis of the saidsurface of the workpiece,

b. supporting and rolling the portions of each of the said rollers oneither axial side of the said median portion in rolling line contactwith massive backup means, the axial length of said line contact beinggreater than the axial length of contact between the workpiece and thesaid median portions,

c. causing relative movement between the workpiece and the rollers aboutthe axis of the said surface of the workpiece, and

d. causing relative movement between the said workpiece and rollers inthe direction of the said axis of the said surface of the workpiece, thearrangement being such that rolling causes considerable metal flow inthe workpiece.

29. A method as claimed in claim 2a in which there is no substantialdeviation from true rolling between the backup means and the saidportions on either side of the median portion of each roller. I

2b. Apparatus for rolling a workpiece having at least one surface whichis a surface of revolution, the apparatus comprising:

a. structure defining a throat and being comprised at least partially bya series of rollers, each said roller having a median portion to engagethe workpiece and additional portions on either axial side of saidmedian portion, and

b. massive backup means on which the said additional portions aresupported and roll in line contact, the axial length of the said linecontact being greater than the axial length of said median portion.

2'7. Apparatus as claimed in claim as in which the diameter of the saidmedian portion is greater than the maximum diameter of the saidadditional portions.

time

1. Apparatus for rolling a tubular workpiece comprising structuredefining an annular throat and within the throat an annular series ofnonspherical rollers, each of which has its center portion of grEaterdiameter than the diameter of its axially outer portions, said throatcomprising at least one backing member having a surface which is asurface of revolution on which surface at least said outer portions ofthe rollers roll in line contact, whereby in use a tubular workpiecepassing through the throat with relative rotation between the workpieceand the rollers about the axis of the workpiece undergoes deformation ofa surface thereof.
 2. The apparatus of claim 1 further comprising meansfor causing relative rotation between the rollers and a workpiece. 3.The apparatus of claim 1 further comprising means for advancing theworkpiece through the throat.
 4. The apparatus of claim 1 wherein therollers are arranged so as in use to act on the outside of theworkpiece.
 5. Apparatus as claimed in claim 4, and a mandrel locatedwithin the throat to support the inside surface of the workpiece. 6.Apparatus as claimed in claim 5, in which the surface of said mandrel iscontoured.
 7. The apparatus of claim 1 wherein the rollers are arrangedso as in use to act on the inside of the workpiece.
 8. Apparatus asclaimed in claim 7, in which the throat-defining structure is contoured.9. The apparatus of claim 1 wherein the rollers are arranged with theiraxes substantially parallel with the axis of the throat.
 10. Theapparatus of claim 1 in which the rollers are arranged with their axesoblique relative to the axis of the throat.
 11. The apparatus of claim 1including a number of annular series of rollers spaced apart along thelength of the throat.
 12. The apparatus of claim 11 in which thediameters of the circles encompassed respectively by the series ofrollers decrease along the length of the throat.
 13. The apparatus ofclaim 1 in which the backing member is formed with an annular groove forsaid series of rollers.
 14. The apparatus of claim 13 in which thebacking member is bored to provide the throat and said groove isprovided within the wall of the bore.
 15. The apparatus of claim 7 inwhich the backing member is a stem adapted to enter the workpiece. 16.The apparatus of claim 15 in which the structure is a housing with acylindrical aperture which encloses the workpiece.
 17. The apparatus ofclaim 15 in which the structure is a series of rollers parallel with andmounted around the workpiece.
 18. The apparatus of claim 14 in which thebacking member is bored to the outer diameter to which the workpiece isto be brought and is counterbored to receive the rollers.
 19. Apparatusfor treating a hollow workpiece, comprising first and second meansdefining between them an annular aperture and an annular series ofrollers in the aperture, said first means including at least one backingmember having a surface which is a surface of revolution on whichsurface at least the axially outer portions of the rollers are supportedin line contact, the workpiece and said first means being axially androtatively movable relative to each other to cause the rollers totraverse a periphery of the workpiece, said rollers being freelysupported on said backing member for orbital movement about said backingmember whereby upon rotative movement of said first means and workpiecerelative to each other, the line of contact between each roller and thebacking member rotates about the periphery of the backing member and asurface of the workpiece undergoes deformation.
 20. Apparatus as claimedin claim 19, said rollers being elongated in their axial direction andhaving a center portion which contacts the workpiece and is of greaterdiameter than the endwise outer portions of the rollers which do notcontact the workpiece, the total length of said line of contact betweensaid backing member and the rollers being greater than the length ofsaid center portion which contacts the workpiece.
 21. Apparatus asclaimed in claim 19, in which said backing member comprises a mandrelthat extends into the workpiece to cause the rollers to act on the innersurface of the workpiece, said second means comprising an elongatedcircumferentially continuous wall that acts on the outer surface of theworkpiece.
 22. Apparatus as claimed in claim 21, in which the mandrel isof a diameter equal to that required of the workpiece, the mandrelhaving a sliding fit in an elongated bore coaxial with thecircumferentially continuous wall and formed in a rigid housing. 23.Apparatus as claimed in claim 20, in which each of the rollers has stubaxles on opposite axial ends of said center portion, the stub axles ofeach roller having a combined axial length greater than that of saidcenter portion, at least the stub axles of the rollers being supportedin line contact on said backing member.
 24. A method of rolling aworkpiece having at least one surface which is a surface of revolution,the method comprising: a. rolling the said surface at high pressure bythe median portion of each of a plurality of rollers, each roller beingarranged with a major component of the direction of its axis parallel tothe axis of the said surface of the workpiece, b. supporting and rollingthe portions of each of the said rollers on either axial side of thesaid median portion in rolling line contact with massive backup means,the axial length of said line contact being greater than the axiallength of contact between the workpiece and the said median portions, c.causing relative movement between the workpiece and the rollers aboutthe axis of the said surface of the workpiece, and d. causing relativemovement between the said workpiece and rollers in the direction of thesaid axis of the said surface of the workpiece, the arrangement beingsuch that rolling causes considerable metal flow in the workpiece.
 25. Amethod as claimed in claim 24 in which there is no substantial deviationfrom true rolling between the backup means and the said portions oneither side of the median portion of each roller.
 26. Apparatus forrolling a workpiece having at least one surface which is a surface ofrevolution, the apparatus comprising: a. structure defining a throat andbeing comprised at least partially by a series of rollers, each saidroller having a median portion to engage the workpiece and additionalportions on either axial side of said median portion, and b. massivebackup means on which the said additional portions are supported androll in line contact, the axial length of the said line contact beinggreater than the axial length of said median portion.
 27. Apparatus asclaimed in claim 26 in which the diameter of the said median portion isgreater than the maximum diameter of the said additional portions.